Short answers, pulled from the story.
A gendèr is a type of metallophone used in Balinese and Javanese gamelan music. It consists of 10 to 14 tuned metal bars suspended over a tuned resonator of bamboo or metal, tapped with a mallet of wooden disks. Each key is a note of a different pitch, often extending a little more than two octaves.
Most gamelans include three gendèr because each is tuned to a different scale and mode. One is for sléndro, one for pélog pathet nem and lima, and one for pélog pathet barang. With five notes per octave in a seven-note pélog scale, some pitches are left out according to the pathet.
The gendèr barung and gendèr panerus both span approximately two and a half octaves, with the panerus pitched an octave higher. In Gamelan Surakarta, the panerus plays a single melodic line similar to the siter, while the barung plays a slower but more complex pattern with separate right and left hand lines that meet at kempyung and gembyang intervals.
The gendèr is tapped with a mallet of wooden disks in Bali or a padded wooden disk in Java, and its two hands sometimes move in parallel but often play contrapuntally. When playing gendèr barung with two mallets, each previously hit note must be dampened by the same hand immediately after new notes are struck, sometimes by holding the mallet at an angle.
The gendèr is similar to the Balinese gangsa, which also has an individual resonator under each key, and the saron, which has tuned metal bars but a single trough resonator. It also resembles the Javanese slenthem, which is pitched lower and has fewer notes.
Cengkok are the semi-improvised patterns that both types of gendèr play, generally elaborating upon the seleh. They are relatively fixed but can be varied to suit the style, pathet, irama, mood, and the skill of the performer, and the cengkok repertoire for gendèr is more developed and specific than that of most other elaborating instruments.